The present invention relates to communications systems and more particularly to a digital radio communications system capable of being employed in a small geographic area, such as a local area network, and in a large geographic area, such as a long distance communications system.
A local area network is a data communications system which links computers, word processors, printers and other related equipment as well as digitally encoded audio signals within a small geographic area. The local area networks allow computers to exchange information. A local area network enables computers to share resources and communicate with computers in other networks, including minicomputers and mainframe computers. In a local area network, information processing devices, such as desktop computers, have access to one another and to hard disk storage, databases, communications software and printers. The use of local area networks has grown at a rapid rate in the past decade as computer systems have become prevalent in the workplace.
Local area networks consist of both hardware and software. In its common form, a local area network has the following elements: (1) network interface units, which functionally and physically connect workstations to the network, may be a printed circuit card inserted into the computer, or may be a separate box attached with a plug to the computer; (2) software, which manages and coordinates the flow of information on the network; (3) file server computer, which acts as the system control unit; and (4) in the past twisted pair wire, coaxial cable or fiber optic cable have been employed to link the components of the local area network.
In a conventional local area network, network interface cards embedded in each computing device control the transmission of data. As originally configured, data is transmitted over coaxial cable, twisted pair wire or fiber optic cable.
In order for a local area network to function properly, all connected devices must obey rules or "protocols" that govern access to the network and control communications with other devices in the local area network. The International Standards Organization (ISO) has developed a framework for network protocols so that incompatible devices will be able to communicate. The ISO model separates the communications and computing services provided by local area networks into seven hierarchical layers. Standards for the two lowest layers in the ISO model, the physical layer and the data link layer, set parameters for media, transmission speed, physical architecture and access method of the local area network. The five higher layers in the ISO model, the software layers, address the methods by which information is reliably transmitted between sending and receiving systems, and the way such information is processed and presented to the user. The data link layer forms a bridge between hardware and software in the local area network. The software layers control the exchange of information.
Local area networks generally use three basic architectures: star, ring and bus. In a star type network, each device is connected with a separate link through a central file server. An example is ATT's STARLAN. In a ring, such as IBM's TOKEN RING, devices are connected to one another in a closed loop. In a bus architecture, devices are attached to a single open-ended cable. Examples are ETHERNET and ARCNET.
While originally local area networks had their computers connected to one another through twisted pair wires, coaxial cables or fiber optic cables, more recently the computers have been connected in a local area network by the means of radio frequency (RF) waveforms. Such a radio communications system for a local area network has been disclosed in U.S. Pat. No. 4,789,983. In this patent there is disclosed a local area network using RF waves rather than wires to connect users to a central node or station. The users timeshare different RF frequencies for transmission and reception. In this local area network, the users operate one at a time, do not employ focused antennas and the interference and multipath fading is overcome by employing diversity techniques.
The communications system of the present invention is not limited to local area networks, but rather can also be used in long distance communications systems of the type employing relay stations for line-of-sight communications over a large geographic area or employing an orbiting satellite as the relay station for the long distance communication system. A satellite communication system is disclosed in U.S. Pat. No. 3,611,435 which discloses a satellite communication system where each terminal of a system transmits data to a satellite repeater or relay station on a carrier signal which has a frequency different for each terminal and each terminal of the system receives data on a carrier signal which is different for each terminal and different for each of the transmitted carrier signals.